Medical tests that require a small volume of blood are well known. For example, test kits for self-measurement of blood sugar levels are utilized by diabetics. These test kits require that a drop of blood be placed on a test strip that is then inserted into a measurement apparatus that displays the glucose concentration in the blood sample. To obtain the drop of blood, the user is supplied with a lancet device, which makes a skin prick, typically in the user's finger. It would be advantageous to minimize the variation between lancing episodes so that sufficient blood is obtained every time and yet the skin is not overly traumatized, since any unnecessary trauma to the skin would result in unnecessary discomfort.
To successfully obtain blood, a piercing device must traverse the skin's various layers to reach the blood vasculature. Human skin is composed of a tough, keratinized squamous epithelium. The outermost layer of skin is known as the epidermis (0.07 to 0.12 mm thick), and has its own distinct layers: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. (For a review about skin, see Tortora and Anagnostakos "Principles of anatomy and Physiology," Harper and Row 1981). Underneath the epidermis is the dermis, which is 1 to 2 mm thick. Because of its varying elasticity and thickness due to the cellular structure and anatomical locations, the force necessary for penetrating the epidermis to access the vascular beds within the dermis layer will vary. It has been reported that skin tension is the greatest in the areas where the epidermal elastic keratinous fibers are dense, particularly in regions where the skin is thick, such as is found in the epigastric (stomach) regions.
To penetrate the skin surface requires a force applied normal to the surface of the skin equal to or greater than the rupture strength of the skin. Below this force level, there is first an elastic range within which the degree of deflection corresponds directly with the applied force (skin depression), followed by a non-linear response by the skin (otherwise known as the inelastic response), corresponding to the further stretching of the skin at the point of application prior to rupture. The applied force reaches a maximum when the skin ruptures, resulting in the penetration of the object into the skin. The capillary bed under the dermis is approximately 300 to 750 microns below the outer surface of the skin in the areas of the fingers, the forearms and the abdomen. Bleeding can occur when the penetration of the object reaches the capillary bed.
Many factors affect the pain sensation associated with the sampling of blood. One factor is thought to be the generation of pressure waves that are built up at the site of puncture. Providing a shallow taper to the piercing end of the lancet should reduce the pressure wave buildup, and thereby reduce the pain sensation of blood sampling.
Another successful method to minimize the pain of skin puncture by a needle, pin or lancet, is to minimize the area over which the puncture occurs. This can be achieved by miniaturizing the needle or lancet and by reducing the force applied to create the wound. The smaller the needle, the less force is required to puncture the skin, and less nerves endings are stimulated by the cut. Unfortunately, as the size of the needle used becomes smaller, the tendency of the needle to flex increases. Flexing the needle during skin penetration will increase the discomfort of the user.
Additional ways to reduce the discomfort associated with blood sampling include reducing the penetration depth of the needle into the skin and rapidly advancing and retracting the needle from the skin. Optimizing the factors to reduce patient discomfort will encourage compliance to self test, for example, for diabetic self-monitoring. Certain lancets and lancet launchers have been used. For example, U.S. Pat. No. 4,976,724 (Nieto, et al.), U.S. Pat. No. 5,318,584 (Lange, et al.); U.S. Pat. No. 5,628,764 (Schraga); U.S. Pat. No. 5,611,809 (Marshall, et al.); U.S. Pat. No. 5,624,458 (Lipscher); U.S. Pat. No. 5,628,765 (Morita); U.S. Pat. No. 5,630,828 (Marwhirt, et al.); U.S. Pat. No. 5,643,306 (Schraga); U.S. Pat. No. 5,645,555 (Davis, et al.); U.S. Pat. No. 5,707,384 (Kim); U.S. Pat. No. 5,733,300 (Pambianchi); and U.S. Pat. No. 5,746,761 (Turchin) disclose reusable or disposable launchers for lancets.
However, for blood sampling, there is still a need for a technique that can provide an adequate amount of blood with little discomfort to a patient and provide less variation in penetration depth among different lancing episodes.